262 ENGINE TESTS. 



economy of condensing is, at best, not much over 20% ; 

 and this is on the basis of steam consumption. If, further- 

 more, we allow for the difference produced by heating the 

 feed-water to the extent mentioned above, the saving of fuel 

 would be reduced to about 11%. In some cases in practice 

 where these conditions exist, the difference in favor of con- 

 densing might be greater, owing to the evaporative economy 

 of the boilers being improved by reducing the work upon 

 them ; but all that could be fairly expected on the basis of 

 these three engines, other things being the same, would be not 

 much over 10%. 



There is another method of looking at this subject; and that 

 is, to compare the best performance of engines running con- 

 densing with the best results running non-condensing. The 

 best non-condensing result, in the list of simple engines using 

 ordinary steam, is that of Engine No. 31 F, working at about 

 100 Ibs. pressure at .231 cut-off, and developing 287.1 indi- 

 cated horse-power. This result is 25.39 Ibs. The best result 

 obtained from a condensing engine, using ordinary steam, is 

 that of No. 22, working at a pressure of 82.3 Ibs. at .172 

 cut-off, the engine developing 613.4 I. H. P. This is 18.49 

 Ibs. per I. H. P. per hour. Comparing these two figures, there 

 is a difference in favor of the condensing engine of 6.9 Ibs., 

 or 27.2% of the larger quantity. Allowing for steam or 

 power used by the condensing apparatus, the net economy 

 in feed-water consumption is not. at best. >ver 25% ; and fur- 

 ther allowance for the gain due to heating the feed-water, as 

 estimated in the former case, would bring the coal-saving down 

 to about 17%. It appears, therefore, that the tests here given 

 on simple engines do not confirm the popular impression that 

 the saving produced by condensing is 2o ' . 



The economy of condensing, as compared with non-con- 

 densing, depends to some extent on the type of air-pump and 

 condenser employed. There are four principal classes of 

 these : 



1. The jet condenser and direct-connected air-pump, which 

 uses power supplied by the main engine. 



